Cuthbert is going to take Karkat's head with him until someone makes him leave it behind or just takes it from him. He's still too numb to do anything but nod at Kankri when he addresses him.

"Fourth floor then, we can do that. Fourth floor and back again."

It's easier to focus on what he can do rather than all the thoughts bouncing around his head. This is how he gets through a crisis, by sticking to the things he knows.